Method to prevent PCB plate warping

- Sep 22, 2018-

1. Why is the circuit board so flat

In the automatic insert line, if the printing plate is not smooth, it will cause inaccurate positioning, and the components cannot be inserted into the hole and surface of the plate, and even break the automatic insert machine.The welded plates with the components are bent, and the component feet are difficult to be cut in order.The boards can't fit into the case or the socket, so it's equally annoying for an assembly plant to get stuck on the board.At present, the PCB has entered the era of surface mounting and chip mounting, and the assembly plant must be more and more strict.

2. Standard and test method for warping degree

According to ipc-6012 (1996 edition) << rigid PCB identification and performance specification >>, the allowable maximum warp and twist for surface mounting of PCB is 0.75%, and other boards allow 1.5%.This is an improvement over ipc-rb-276 (1992 edition) in the requirement for surface mounting of the PCB.Currently, the warping degree approved by each electronic assembly plant, whether double-sided or multilayer, is 1.6mm thick, usually 0.70 ~ 0.75%, and many SMT, BGA boards require 0.5%.Some electronics factories are agitating to increase the standard of warpage to 0.3% and test the warpage using methods such as gb4677.5-84 or ipc-tm-650.2.4.22b.Put the printing plate on the platform to be checked, and insert the test pin into the place with the maximum warp.

3. Anti-plate warping in the manufacturing process

1. Engineering design: matters needing attention in the design of printed boards:

A. The arrangement of semi-solidified sheets between layers should be symmetrical, for example, six layers, the thickness between 1 ~ 2 and 5 ~ 6 layers should be the same as the number of sheets of the semi-cured sheets, otherwise the lamination will be prone to warp.

B. Multi-layer core board and semi-cured sheet shall use the same supplier's product.

C. The graphic area of the circuit on the outer surface A and B should be as close as possible.If face A is A large copper surface, while face B only walks A few lines, this kind of printing plate is easy to warp after etching.If the two sides of the line area difference is too large, the thin side can be added some independent grid to balance.

2. Baking sheet before blanking:

The purpose of drying the copper clad sheet (150 degrees Celsius, 8 + 2 hours) is to remove the water in the sheet and to allow the resin in the sheet to fully solidify, further eliminating the remaining stress in the sheet, which is helpful to prevent the plate from warping.At present, many double-sided, multi-layer board still adhere to the pre - or post-drying plate.But there are also some plate factory exceptions, at present, PCB factory drying plate time provisions are inconsistent, from 4-10 hours are available, it is suggested that according to the production of the printed board class and customer requirements on warp.Both methods are feasible, and it is suggested to dry the sheet after cutting.The inner plate should also be baked.

3. Longitude and latitude of semi-cured tablets:

The warp and weft shrinkage rates of semi-cured sheet after lamination are different.Otherwise, after the lamination is easy to cause the finished plate warping, even if the pressure drying plate is difficult to correct.Many of the reasons for the warping of multilayer plates are due to the fact that the laminates on the laminates are not clearly separated from the latitude and longitude of the semi-cured sheets.

How to distinguish longitude and latitude?The rolled semi-curing sheet is rolled in meridional direction and widths in latitudinal direction.For copper foil, the long edge is zonal and the short edge is meridional. If not sure, you can contact the manufacturer or supplier.

4. De-stress after lamination:

After the completion of hot and cold pressing, the laminates are taken out, sheared or milled off the burrs, and then laid flat in the oven at 150c for 4 hours to gradually release the stress in the plate and allow the resin to fully solidify. This step cannot be omitted.

5. The plate shall be straightened when electroplated:

Special clamping rolls should be made when 0.4 ~ 0.6mm ultra-thin multilayer plates are used for plate surface plating and pattern plating. After the thin plates are clamped on the fly bar of automatic plating line, a round rod is used to string up the clamping rolls on the whole fly bar, so as to straighten all the plates on the roller, so that the plates after plating will not be deformed.If this measure is not taken, after electroplating the copper layer of 20 or 30 microns, the thin plate will be bent and difficult to remedy.

6. Cooling of plates after hot air leveling:

The hot air of the printed board is impacted by the high temperature of the solder groove (about 250 degrees Celsius) at ordinary times. After taking out, it should be put on the flat marble or steel plate for natural cooling and then sent to the post-processing machine for cleaning.This is good for warping the board.In some factories, to enhance the brightness of the surface of the lead tin, the hot air of the board is immediately put into cold water after leveling, and a few seconds later is taken out for post-processing.In addition, air float bed can be added to the equipment for cooling.

7. Treatment of warping boards:

Manage orderly factory, PCB will be inspected for 100% flatness at final inspection.All unqualified boards will be picked out, put into the oven, at 150 degrees Celsius and under pressure for 3 ~ 6 hours, and naturally cooled under pressure.Then remove the plate and check its flatness. This can save part of the plate.The air pressure plate warping and anti-straightening machine of Shanghai huabao agent has very good effect in repairing the warp of the circuit board.If the above mentioned anti-warping process measures are not implemented, part of the plate baking pressure is useless, can only be scrapped.